The present invention relates to a printer such as a dot matrix printer.
Wire-dot-impact printers have a carriage slidably movable on a guide rod along a platen, a print head for projecting and retracting print wires from and into its distal end, and an ink ribbon. A sheet of print paper is set on the platen and fed as the platen rotates about its own axis. Then, the print wires are selectively controlled to strike the sheet of print paper through the ink ribbon to thereby print desired information on the sheet of print paper. In order to achieve appropriate printing in such printers, it is necessary that a suitable gap be present between the sheet of print paper and the distal end (print wires) of the print head.
Recently, there has been developed a printer which incorporates an adjusting mechanism for automatically adjusting the gap between the sheet of print paper and the print head. The adjusting mechanism includes a pressing/releasing mechanism for moving the carriage toward and away from the platen through angular movement of the guide rod about an axis which is displaced eccentrically off the geometric central axis of the guide rod. Before a printing process is carried out, a ribbon mask on the distal end of the carriage is pressed under predetermined pressure against the sheet of print paper on the platen by the pressing/releasing mechanism, and thereafter the carriage is moved a suitable distance away from the platen by the pressing/releasing mechanism.
When the printer is shipped from the factory, a control device comprising a microcomputer or the like, which controls the printer, stores, as a reference position, the position of the carriage at the time the ribbon mask is pressed against the platen under a pressure which is the same as the above predetermined pressure. The thickness of the sheet of print paper is detected as the difference between the reference position and the position of the carriage when the ribbon mask is pressed against the sheet of print paper. The carriage is displaced from the sheet of print paper by a preset distance which corresponds to the detected thickness of the sheet of print paper. Therefore, the print head can print desired information on any of various sheets of print paper having various thicknesses, while being spaced from the sheet of print paper by a distance suitable for printing.
The platen is driven to rotate about its own axis by a motor and hence serves as a sheet feed mechanism. However, the platen is not constructed with high accuracy for reasons in connection with the manufacture thereof, and may be installed such that the geometric central axis thereof is displaced, though slightly, off the axis (rotational axis) about which the platen is rotatable. If the platen is thus eccentrically positioned, then different positions on the circumference of the platen are spaced different distances from the rotational axis of the platen.
The conventional adjusting mechanism, as described above, is based on the assumption that the circumferential surface of the platen is of a true circle spaced equally from the rotational axis, and regards, as the reference position, the position of the carriage at the time it is pressed against the platen at a certain position thereon. Therefore, if the geometrical central axis of the platen is displaced off the rotational axis, then the following problems arise: When the carriage is pressed against a circumferential area of the platen which is spaced a larger distance from the rotational axis thereof than other circumferential areas of the platen in order to determine the reference position for the carriage, the reference position which is determined is located closer to the carriage than a standard circumferential surface of the platen, which is spaced a standard distance from the rotational axis of the platen. If the carriage is spaced a certain distance from that reference position, then the gap between the sheet of print paper and the print head is so large that the density of printed information on the sheet of print paper may be too low or some dots to be printed may not be printed on the sheet of print paper.
On the other hand, when the carriage is pressed against a circumferential area of the platen which is spaced a smaller distance from the rotational axis thereof than other circumferential areas of the platen in order to determine the reference position for the carriage, the reference position which is determined is located radially inwardly of the standard circumferential surface of the platen. If the carriage is spaced a certain distance from that reference position, then the gap between the sheet of print paper and the print head is so small that the density of printed information on the sheet of print paper may be too high or the ink ribbon may be caught by the print head.
The conventional adjusting mechanism, therefore, may fail to establish a reference position with respect to the standard circumferential surface of the platen. As a consequence, the print head may not be spaced a suitable gap from the sheet of print paper on the platen, resulting in a reduction in the print quality.